Nanofabricated volume gratings

ABSTRACT

Methods to manufacture volume transmission diffraction grating (quasi-Bragg gratings) and volume transmission diffraction gratings made by those methods.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 14/216,999, entitled NANOFABRICATED VOLUME GRATINGS, filed onMonday Mar. 17, 2014, now U.S. Pat. No. 10,955,596, which claimspriority from U.S. Provisional Patent Application Ser. No. 61/800,359,filed Mar. 15, 2013, entitled NANOFABRICATED VOLUME GRATINGS, both ofwhich are incorporated herein by reference in their entirety and for allpurposes.

STATEMENT OF GOVERNMENT INTEREST

This invention was made with U.S. Government support from the U.S. AirForce under Contracts FA8650-04-M-1653 and FA8650-05-C-1816, and theU.S. Army under Contract W15P7T-05-C-F001. The U.S. Government hascertain rights in the invention.

BACKGROUND OF THE INVENTION

This invention relates generally to volume transmission diffractiongrating (quasi-Bragg gratings).

A number of devices, such as hyperspectral imagers, use high efficiencyand broad spectral bandwidth transmission diffraction gratings and theperformance of the device is dependent on the use of use high efficiencyand broad spectral bandwidth transmission diffraction gratings.

There is a need for methods to manufacture volume transmissiondiffraction grating (quasi-Bragg gratings) and for volume transmissiondiffraction gratings made by those methods.

SUMMARY OF THE INVENTION

The various embodiments of the present teachings disclose methods tomanufacture volume transmission diffraction grating (quasi-Bragggratings) and volume transmission diffraction gratings made by thosemethods.

For a better understanding of the present invention, together with otherand further objects thereof, reference is made to the accompanyingdrawings and detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustratively shown and described in referenceto the accompanying drawings, in which:

FIG. 1 shows a cross section of a lithographically fabricated volumetransmission diffraction grating;

FIGS. 2A and 2B show a photolithographic mask layout at differentmagnifications;

FIG. 3 shows a photomicrograph of a lithographic mask written usingholographic techniques;

FIGS. 4A and 4B show a photomicrograph of a lithographic mask writtenusing holographic techniques at different magnifications;

FIGS. 5A, 5B, 5C show a series of photomicrographs tracing the majorsteps in the lithographic fabrication of a 10 micron period and a 70%clear duty cycle grating;

FIGS. 6A, 6B and 6C show a series of photomicrographs tracing the majorsteps in the lithographic fabrication of a 20 micron period and a 10%clear duty cycle grating;

FIGS. 7A, 7B and 7C show a series of photomicrographs tracing the majorsteps in the lithographic fabrication of a 20 micron period and a 40%clear duty cycle grating;

FIGS. 8A, 8B and 8C show a series of photomicrographs tracing the majorsteps in the lithographic fabrication of a 20 micron period, 90% clearduty cycle grating;

FIG. 9 is a view of an array of 16 transmission gratings on a 4-inchsilicon wafer;

FIG. 10 is a SEM photomicrograph of a deep-etched transmission gratingin silicon;

FIG. 11 shows a top view of a fused silica grating test sample;

FIGS. 12A and 12B show micrographs of the structure in a 2 μm spatialperiod grating;

FIG. 13 is a micrograph of a wide-open duty cycle fused silica grating;

FIGS. 14A and 14B show different magnification micrographs of a fusedsilica grating with a wider duty cycle;

FIG. 15 shows a micrograph of a residual aluminum mask on a fused silicagrating;

FIG. 16 is a sectional view showing the operation of a volumetransmission diffraction grating;

FIG. 17 is a cross-sectional photomicrograph of a cleaved silicongrating;

FIG. 18 shows steps of germanium grating fabrication using LPCVD;

FIG. 19 shows steps of germanium grating fabrication using a siliconmold;

FIG. 20 shows finishing steps for gratings fabricated using a siliconmold;

FIG. 21 shows a filled silicon transmission grating array;

FIG. 22 shows a close-up of the cross section of the poly-silicon-filledlarge open-duty-cycle silicon grating;

FIG. 23 shows a close-up of the cross section of the poly-silicon-filledmedium open-duty-cycle silicon grating;

FIG. 24 shows a cross-section of a 20 μm period poly-silicon-filledsmall open-duty-cycle silicon grating;

FIG. 25 shows a bright-field image of a 10 μm period poly-silicon-filledsilicon grating;

FIG. 26 shows a darkfield image of a 10 μm period poly-silicon-filledsilicon grating;

FIG. 27 shows a sample silicon grating cleaved from a grating arraywafer;

FIG. 28 shows a small fused silica test chamber in a furnace; and

FIG. 29 shows a 20 μm period grating structure molded into germanium.

DETAILED DESCRIPTION

The present invention is introduced using examples and particularembodiments for descriptive purposes. Although a variety of examples arepresented to show how various configurations can be employed to achievethe desired improvements, these particular embodiments are onlyillustrative and not intended in any way to restrict the inventionspresented.

The novel compact high performance transmission grating basedhyperspectral imagers developed in this effort are enabled by highefficiency and broad spectral bandwidth transmission diffractiongratings. This performance has been shown feasible and demonstrated inthe NIR and SWIR bands in this effort. In order to obtain this level ofperformance in transmission gratings for the MWIR and LWIR bands,lithographic techniques were used to fabricate volume transmissiongratings. In this approach, alternating regions of high and low indexmaterial were lithographically etched or deposited to produce refractiveindex variations throughout a volume (typically on the order of 10wavelengths thick) using non-absorbing or slightly absorbing materialsfor the spectral bands of interest. This type of grating is illustratedin FIG. 1 .

Annealing and similar techniques may be used to smooth the refractiveindex profile and some materials and geometries may even result insinusoid-like modulation profiles. However, refractive index profileswith some degree of stepped- or square-wave-characteristics are easy toproduce, easily controlled, and inexpensive. For these reasons, it wasimportant to develop an accurate model for volume phase gratings withvarying degrees of a stepped modulation profile. This model was used toguide the initial material and parameter selections for the feasibilityexperiments. The following section describes the progress made duringthis effort to expand the thin grating decomposition theory to includesquare wave and other refractive index profiles.

FIG. 1 shows a lithographically fabricated volume transmissiondiffraction grating 10. Alternating material regions of high and lowrefractive index are fabricated making use of lithographic masking,etching, and deposition techniques.

The grating thicknesses are typically on the order of ten wavelengths,and often the grating profiles are characterized by square-wavemodulation.

As discussed in Sections reference to figures below, it was boththeoretically and experimentally demonstrated that polymer-dispersedliquid crystal (PDLC)-based volume transmission gratings can meet thehigh efficiency and broad spectral bandwidth requirements for the WRIcompact, lightweight, visible — SWIR (0.5-1.7 micron) hyperspectralimaging sensor. These PDLC-based gratings, however, are limited in theextent of their spectral transmission bands due to the transmissioncharacteristics of the polymer matrices. Lithographically generatedvolume transmission gratings, which can be produced using standardlithographic techniques, are limited only by the transmissioncharacteristics of the substrate material, and can therefore be extendedfor use in the MWIR, LWIR, and other spectral bands. For example, agrating structure may be written on a lithographic mask and transferredinto fused silica, sapphire, gallium arsenide, silicon, germanium, andperhaps other substrate materials such as barium titanate, magnesiumfluoride etc. using well characterized processes such as reactive ionetching. In this phase of the effort volume transmission gratings werelithographically produced in SiO₂ substrates. The feasibilitydemonstration of this technique is extended further into the infraredwith lithographically fabricated gratings in silicon.

Conventional Lithographic Masks

A 5 inch square chrome-on-quartz mask containing a 4×4 array of 16square wave binary gratings, each 15 mm square, and each with variationsin spatial frequency and duty cycle was designed at the CornellNano-Scale Science & Technology Facility (a member of the NationalNanofabrication Users Network) and is illustrated in FIGS. 2A and 2B.This 5 inch square chrome-on-quartz mask contains a 4×4 array of 16gratings, each of which is 15 mm square, and contains a unique spatialfrequency and duty cycle combination. A detailed blow-up of a singlegrating region is illustrated in FIGS. 2A and 2B.

This pattern was then etched into the selected substrate to a depthwhich represents the thickness L of the grating using lithographictechniques. This thickness is an important variable in the optimizationof the spectral bandwidth and efficiency of the grating. The duty cyclevariation is useful for effectively tuning the refractive indexmodulation, which can alternatively be accomplished by filling theetched grating with other materials. This mask was used to exposephotoresist on transparent transmission grating substrates such as fusedsilica and silicon, which was then transferred into the substrate usingtechniques such as reactive ion etching. The resulting diffractiongratings are designed to operate in the volume (quasi-Bragg) diffractionregime yielding high peak diffraction efficiencies and broad spectralbandwidths.

FIGS. 2A and 2B show a photolithographic mask layout. FIG. 2A shows a 5inch square chrome-on-quartz mask 52 containing a 4×4 array of 16gratings 56, 58, 60, 62, each measuring 15 mm square with variations inspatial frequency and duty cycle, was fabricated and is shown. This maskwas used to expose photoresist on transmission grating substrates suchas fused silica and silicon, which were then transferred into thesubstrate using techniques such as reactive ion etching. The resultingdiffraction gratings are designed to operate in the volume (quasi-Bragg)diffraction regime yielding high peak diffraction efficiencies and broadspectral bandwidths. FIG. 2B shows a magnified section of thephotolithographic grating mask pattern illustrated in FIG. 2A andreveals the 20 micron spatial period of the grating. In this magnifiedview, the darker regions 68 represent chrome (opaque) areas, while thelighter regions 66 represent transparent areas.

Holographically Written Lithographic Masks

The benefits of writing the lithographic mask using holographictechniques were also demonstrated in this effort. In this technique,photoresist is deposited on a quartz substrate that has a thin aluminumfilm evaporated on it. The photoresist coated wafer is thenholographically exposed and the photoresist is then processed. Followingthe photoresist processing, the photoresist pattern is transferred intothe aluminum mask layer using ion etching. This gives rise to a verypure, high contrast, and ghost-free mask that can be used to patternsubstrates with the aspect ratios desired for high-performancequasi-Bragg regime volume transmission gratings. FIG. 3 is aphotomicrograph of a thin aluminum film 80 on a SiO₂ substrate andillustrates the purity and ghost free characteristics of theholographically written grating lines. The aluminum film waslithographically etched using holographically exposed photoresist, asdescribed above. This aluminum mask was then used to etch the gratingpattern into the quartz substrate using a PlasmaTherm 72 Reactive IonEtching System.

FIG. 3 shows a photomicrograph of a lithographic mask written usingholographic techniques. A thin aluminum film was lithographically etchedusing holographically exposed photoresist, which was then used to etchthe grating pattern into a SiO₂ substrate using reactive ion etching.This photomicrograph shows the purity and ghost free characteristics ofthe grating lines. For visible through SWIR spectral bands, the benefitsof writing the mask holographically include low cost and a very pure andghost-free mask pattern that can be used to pattern substrates with theaspect ratios desired for high-performance quasi-Bragg regimetransmission gratings.

For visible through SWIR spectral bands, the benefits of writing themask holographically include low cost and a very pure and ghost-freemask pattern that can be used to pattern substrates with the aspectratios desired for high-performance quasi-Bragg regime volumetransmission gratings. This holographically written mask technique canalso be used for gratings designed to operate in the MWIR and LWIRspectral bands, but for transmission MWIR and LWIR gratings, thetypically larger grating periods required make the conventionallywritten lithographic contact masks described previously a good choice aswell.

An additional lithographically fabricated volume transmission grating ona SiO₂ wafer is shown in FIG. 4A. This grating 102 was one of an arrayof gratings that were simultaneously etched into a SiO₂ wafer 103 andthe edge 101 of the transparent wafer is visible in the photograph dueto scattered light. A larger region of the same SiO₂ wafer 103 showingtwo gratings 104, 106 side-by-side is shown in the photograph of FIG.4B. The illuminating light is diffracted by the lower grating 104, andthe outline of the upper grating 106 is visible.

Lithographically Produced IR Volume Transmission Gratings

The feasibility of lithographically fabricated gratings for use in thevisible, NIR, and SWIR bands was demonstrated with gratingslithographically fabricated in SiO₂ wafers. In this section, thefeasibility of lithographically fabricating the deep-etched structuresrequired for MWIR and LWIR transmission gratings is demonstrated bylithographic fabrication of volume transmission gratings in siliconsubstrates.

Lithographic Fabrication of Silicon Transmission Gratings

To demonstrate the feasibility of lithographically producing the deepetched profiles, an array of 16 gratings was lithographically etchedinto 4-inch diameter silicon wafers. The gratings each measured 15 mm ona side, and were had various combinations of spatial period and dutycycle, which are tabulated in Table 1. The varied grating duty cycle isone way to vary the effective refractive index modulation of thegratings, which directly affects the efficiency and bandwidth of thegratings.

Grating Spatial Grating Number Period Duty Cycle 1 10 μm 20% 2 10 μm 30%3 10 μm 40% 4 10 μm 50% 5 10 μm 60% 6 10 μm 70% 7 10 μm 80% 8 20 μm 10%9 20 μm 20% 10 20 μm 30% 11 20 μm 40% 12 20 μm 50% 13 20 μm 60% 14 20 μm70% 15 20 μm 80% 16 20 μm 90%

Table 1 Transmission Grating Spatial Period and Duty Cycle Combinations.

The lithographic fabrication was performed using the Cornell Nano-ScaleScience & Technology Facility (a member of the National NanofabricationUsers Network) where first a chrome on glass mask 120 was designed asdescribed in previously, and is illustrated in the photomicrograph ofFIG. 5A. Next, photoresist was spin-coated onto the silicon wafers,which were then exposed through the chrome-on-glass mask using contactlithography with a Karl Zeuss MA 6 Contact Aligner. After developing theexposed photoresist, a relief image 130 of the photoresist lines wasleft on the silicon substrate (FIG. 5B), which was then transferred intodeep etched structures 140 (FIG. 5C) by means of a Unaxis SLR 770 ICPDeep Silicon Etcher, which is a Bosch fluorine process inductivelycoupled plasma reactive ion etcher.

The series of photomicrographs 120, 130, 140 traces the major steps inthe lithographic fabrication of a 10 micron period, 70% clear duty cyclegrating, showing: (a) The chrome mask 120 with its alternating clear 122and opaque 124 regions (with a duty cycle of 70% clear, 30% opaque) ofchrome on glass; (b) The developed photoresist layer 130, which is arelief of photoresist lines on top of a silicon wafer; and (c) Theetched silicon wafer 140.

Figures FIGS. 6A-8C contain related series of photomicrographsillustrating the major steps in the lithographic fabrication oftransmission gratings with a spatial period of 20 microns and with 10%clear duty cycles (FIGS. 6A-6C), 40% clear duty cycles FIGS. 7A-7C), and90% clear duty cycles (FIGS. 8A-8C). It is apparent in the limiting caseof 90% clear duty cycles that the resulting very thin photoresist linesand narrow silicon walls are near the high-aspect-ratio limit of thesefirst runs. It is expected that the further optimization of exposure anddevelopment processes will permit future fabrication of theselimiting-case narrow and high-aspect-ratio structures.

The series of photomicrographs of FIGS. 6A-6B traces the major steps inthe lithographic fabrication of a 20 micron period, 10% clear duty cyclegrating, showing: (a) The chrome mask 150 with its alternating clear 152and opaque regions 154 (with a duty cycle of 10% clear, 90% opaque) ofchrome on glass; (b) The developed photoresist layer 160, which is arelief of photoresist lines on top of a silicon wafer; and (c) Theetched silicon wafer 170.

The series of photomicrographs of FIGS. 7A-C traces the major steps inthe lithographic fabrication of a 20 micron period, 40% clear duty cyclegrating, showing: (a) The chrome mask 180 with its alternating clear 182and opaque 184 regions (with a duty cycle of 40% clear, 60% opaque) ofchrome on glass; (b) The developed photoresist layer 190, which is arelief of photoresist lines on top of a silicon wafer; and (c) Theetched silicon wafer 200.

The series of photomicrographs of FIGS. 8A-C traces the major steps inthe lithographic fabrication of a 20 micron period, 90% clear duty cyclegrating, showing: (a) The chrome mask 210 with its alternating clear 212and opaque 214 regions (with a duty cycle of 90% clear, 10% opaque) ofchrome on glass; (b) The developed photoresist layer 220, which is arelief of photoresist lines on top of a silicon wafer; and (c) Theetched silicon wafer 230.

Lithographic Results

The resulting transmission gratings are illustrated in FIG. 9 which is aphotograph of a silicon wafer 250 that contains an array of 16transmission gratings 260. Each of these 16 transmission gratings 260measures 15 mm on a side and has a different combination of spatialfrequency and duty cycle as tabulated in Table 1. This wafer is one ofmultiple wafers that were fabricated, each with a different etch depth.

The photomicrographs of the resulting gratings shown FIGS. 5C, 6C, 7Cand 8C, do not reveal the depth of the etched grating, although whentaking the photomicrographs it was apparent that deep structure existeddue to the large depth over which modulation was seen when focusing. Amore clear indication of the depth of the grating structure is given inthe scanning electron micrograph (SEM) shown in FIG. 10 . This SEM wastaken from an oblique angle above the surface of the wafer 270 near theedge of a 10 micron period grating 272. Following the sidewall 278 downleads to the bottom edge 280 of the grating, from which it is clear thatthe etched grating is indeed deep, and that the grating structureexhibits very large aspect ratios. These gratings demonstrate thefeasibility of using lithographic techniques for the fabrication ofdeep, high aspect ratio structures required for the high efficiency,broad bandwidth IR transmission gratings modeled in this effort.

The feasibility of lithographically fabricating the deeply etchedstructures required for the volume transmission gratings has beenclearly demonstrated in the fabrication runs described above. A seriesof rough measurements were made using a CO₂ laser at a wavelength of10.6 microns and one of the 10 micron spatial period gratings describedabove. These preliminary measurements averaged more than twice thetransmitted energy in the first order beam than in the zero-order beam,and little apparent energy in other transmitted orders. Thus, the volumetransmission grating appears to be operating in the desired quasi-Braggdiffraction regime.

Volume Transmission Grating Experiments

Initial Fused Silica Volume Transmission Diffraction Grating Progress

These feasibility experiments were performed at Cornell University'sCenter for NanoScale Science Technology Facility (CNF), where severalfused silica wafers were coated with photo resist and exposed to themasks generated during the Phase I effort using a 5X stepper in order toallow for the fabrication of smaller grating periods. Since this priormask contained regions with spatial periods of both 10 μm and 20 μm in avariety of duty cycles, the 5-X reduction from the stepper resulted inthe exposure of the photo resist to grating periods of 2 and 4 μm,respectively. After developing the photo resist, a one third of a micronthick layer of aluminum was deposited over the structure and a liftoffprocess was used to produce aluminum-on-fused-silica grating masks.These samples were then put in a reactive ion etcher to produce thefused silica gratings, a photograph of which is shown in FIG. 11 .Scanning electron micrographs (SEMs) were taken at various points acrossthe sample to characterize the etching process.

FIG. 12A illustrates an SEM 350 of one of the resulting narrow-open dutycycle fused silica grating 352. The 2 μm grating 352 structure shownillustrates that a large aspect ratio with steep side walls 354 canstill be fabricated with reduced periods. The side walls are taperedslightly outwards, which is typical until the process is fullyoptimized. Spurious structures are observed in the substrate 358, whichis due to sputtering from the aluminum mask. In this process, specks ofsputtered aluminum act as secondary masks as the substrate is etched.However, this secondary structure may be more than simple masksputtering. FIG. 12B illustrates an etch depth of 4.49 μm for thegrating structure 362, where due to the angle of projection the actualdepth is approximately twice the apparent etch depth shown, orapproximately 9 μm.

A micrograph of a fused silica grating 372 with a wide-open duty cycleis shown in FIG. 13 , where, it can be seen that the spurious structuresextend inside the grating structure as well 374. An etch depth of 4.25μm is illustrated for the grating structure, but again due to the angleof projection, the actual depth is approximately twice the apparent etchdepth shown, or approximately 8.5 μm. This slightly smaller etch depthfor the wider open duty cycle grating indicates that etch depth isdependent on the width of the structure being etched. As a result, theoptimization trades are dependent on the duty cycle of the structure andpossibly on the location of the grating sample on the wafer as well.

A more even aspect ratio grating is presented in FIGS. 14A and 14B underboth low and high magnifications, respectively. Not only is the grating381 plainly visible, but it is surrounded by a field 384 of the spuriousstructure. It is important to eliminate the spurious structure since itmay cause optical scatter in the spectrometer, resulting in both spatialand spectral cross-talk. Spurious structures such as these are commonand they can be eliminated with further process optimization. Due to themasking procedure used in this process, the grating structure risesabove the background. This is in contrast to earlier grating runs wherethe grating structure was etched below the substrate.

Upon close inspection, the micrograph of FIG. 15 reveals that there isapproximately one to two tenths of a micron aluminum mask 393 left onthe grating surface 392. This is an encouraging result since once theside walls 397 are made more vertical, it suggests that much larger etchdepths can be obtained using this mask and etch technique.

High Efficiency LWIR Volume Transmission Gratings

Volume Transmission Grating Design

A key enabling technology for the high performance compact hyperspectralimaging sensors developed in this effort is the high efficiency LWIRvolume transmission diffraction grating. Volume transmission gratingsand their theory of operation are well known, but still represent aniche area in comparison with the more common blazed reflectivegratings. When the operation of these gratings is based on phasemodulation resulting from a volume variation of refractive index, theseelements exhibit diffraction efficiencies near 100% with broad spectralbandwidths. The key to optimizing the performance of these elements forspectrometer applications is to balance the thickness L, period A, andwavelength X appropriately in order to operate the gratings in thequasi-Bragg regime (FIG. 16 ). This occurs when the product of thewavelength and grating thickness are roughly equal to the square of thespatial period of modulation as given by Stone and George (T. Stone andN. George, “Wavelength Performance of Holographic Optical Elements,”Applied Optics, Vol. 24, p.3797, 1985; and T. Stone and N. George,“Bandwidth of holographic optical elements,” Optics Letters, Vol. 7, p.445, 1982). When operating in this regime, the peak efficiency dropsonly slightly from 100% while the spectral bandwidth becomes extremelybroad. These types of volume transmission gratings have been extensivelyfabricated for the visible and near IR spectral regions usingholographic techniques and volume phase recording materials.

FIG. 17 is a photomicrograph 450 of a cleaved cross section 452 from adeeply etched lithographically fabricated silicon diffraction grating.The wafer is 500 μm thick, and the process used to etch these structureshas been used to etch structures with sharp side walls at depths ofhundreds of μm and even entirely through 500 μm wafers. As the aspectratio of these structures is increased, there is eventually somedegradation to the sharpness of the profile, but from thephotomicrograph FIG. 17 , it can be seen that none of these effectsoccurred for the aspect ratios used in this effort.

Deposition of Germanium on Silicon Grating Structures

In an alternative approach, the deeply etched silicon grating can beused to form high aspect ratio germanium gratings through the germaniumdeposition process illustrated FIG. 18 . In this process, a siliconwafer 462 is first etched to the desired spatial period and depth. Then,instead of depositing poly-silicon as described previously,poly-germanium 488 can be deposited. This should be readily accomplishedusing Low Pressure Chemical Vapor Deposition (LPCVD), for example, whichwith germanium is accomplished at even lower temperatures than withpoly-silicon. After the germanium deposition, the silicon 482 can thenbe dissolved with one of a variety of solvents that are selective tosilicon and do not attack the germanium 488, leaving behind thegermanium grating structure 498.

To tune the refractive index modulation, this germanium grating can besubsequently filled with a material of differing refractive index suchas, for example, poly-crystalline or amorphous germanium that has adifferent refractive index than that of the host grating. This issimilar to what was done in the filled silicon grating feasibilityexperiment described below. As with poly-silicon deposition, the crystalsize of the poly-germanium (ranging from coarse crystal domains towardthe amorphous limit) can be varied with deposition temperature and canprovide a fine tuning mechanism for the final grating refractive indexmodulation for optimizing efficiency and spectral bandwidth.

Molding of Germanium on Silicon Grating Structures

Additional LWIR volume transmission grating fabrication techniques weredeveloped, which were based on using mass-producible deep-etched silicongratings as molds for an inexpensive molding process. The electronmicrograph of FIG. 10 illustrates the deep, high aspect patterning thatis readily achievable using standard silicon processing techniques.These deep-etched silicon structures can be used as a mold forfabricating gratings from materials other than silicon. Silicon is anideal mold material due to its etching properties, very low cost, andthe abundance of high yield patterning processes which are alreadydeveloped.

The molding process developed during the Phase I effort for fabricatingLWIR volume transmission diffraction gratings is illustrated as a threestep process in FIG. 19 . In this molding process, a lump 504 of desiredgrating material (germanium in this case) is first placed on a patternedsilicon grating mold 512 as shown in Step 1. These components are nextplaced in a vacuum furnace, where the fill material is melted as shownin Step 2. The vacuum environment prevents spurious reactions includingthe formation of any oxides or nitrides. As shown in Step 3, the vacuumenvironment also allows for the forced filling 536 of the deep trenches,and depending on the surface tensions and wetting properties of thematerials, the vacuum furnace chamber may also be backfilled with aninert gas to force the molten fill material conformably into the mold.

At this stage, there are a number of optional finishing steps, eachresulting in differing grating structures. FIG. 20 illustrates threeexamples of these finishing “fourth steps” that follow the three moldingsteps illustrated in FIG. 19 . In step 4a, the silicon mold is dissolvedaway, leaving only the deep etched germanium grating 542. Excellentsilicon-selective solvents exist for this purpose. This pure germaniumgrating 542 can be used “as is,” or as a germanium mold for anothermolding cycle in which a third material is filled into the germaniumgrating to provide the desired transmission band and refractive indexvariation.

Another option, shown as step 4b in FIG. 20 , illustrates the result ofthe mechanical grinding-away and polishing of the base of the siliconmold, resulting in a silicon-filled germanium grating 552. For systemsoperating in the LWIR spectral band, the absorption effects of thesilicon filling are negligible, especially where narrow duty-cycles areused. For example, if a 90%-open duty cycle is used in the silicon mold,then only 10% of the modulated region of the germanium grating willcontain silicon, which itself only partially absorbs in the LWIR band.Furthermore, for typical modulation depths of approximately 100 μm orless, the resulting absorption of the silicon would be on the order ofonly 0.5% for the 9 μm absorption band, which is likely to beinsignificant to the overall system performance.

In another variation shown in step 4c of FIG. 20 , the silicon-filledgermanium grating is annealed, forming a spatially-varying alloyedsilicon-germanium volume transmission grating. Silicon-germanium alloysare in wide use in the microelectronics industry, and other combinationsof materials may also be readily used with the techniques describedhere. In addition to resolving stresses in the modulated structure, thisprocess can lead to a more smoothly varying (if not nearly sinusoidal)refractive index profile in place of the more stepped square-wavemodulation typical of lithographically fabricated volume transmissiongratings. Further, this annealing process can be used to provide a verysensitive control of the refractive index modulation for the volumetransmission grating that will be useful in optimizing the peakefficiencies and the spectral bandwidths of the resulting volumetransmission gratings. The molding techniques described here can also beapplied to other filling and annealing techniques such as sputtering,evaporating, and CVD, but these molding techniques hold the promise ofvery low cost and high quality LWIR transmission grating fabrication.

Feasibility Experiments Supporting Fabrication by Deposition

Fabrication Overview

The feasibility of fabricating LWIR volume transmission gratings byfilling lithographically fabricated diffraction gratings with materialsof varying refractive index was experimentally demonstrated. Thisfilling process represents an important asset in the design of thesetransmission gratings since it provides a method of fine control overthe refractive index modulation for optimizing the peak diffractionefficiency and spectral bandwidth of volume transmission gratings.

In a related effort, arrays of silicon volume transmission gratings weresuccessfully made to demonstrate the broad feasibility of fabricatinghigh performance transmission diffraction gratings for the Visible, NIR,SWIR, MWIR, and LWIR spectral bands using lithographic techniques. Inthe feasibility experiment for LWIR gratings, some of these deeplyetched silicon grating arrays were filled with poly-silicon materialusing a low pressure chemical vapor deposition (LPCVD) techniqueperformed using the facilities of the Cornell Nanoscale Science andTechnology Facility at Cornell University (NNF-CNF). A photograph of oneof these poly-silicon-filled diffraction grating arrays is shown FIG. 21.

In FIG. 21 , gratings 610 in the silicon wafer 600 were “filled” withpoly-silicon using low pressure chemical vapor deposition (LPCVD). Thepoly-silicon has a refractive index that can be varied from that of thecrystalline silicon substrate by controlling the deposition parameters.This and related filling techniques provide the ability to tune therefractive index modulation of the volume transmission gratings in orderto optimize peak diffraction efficiency and spectral bandwidth. Thefabrication and filling of these gratings was performed using thefacilities of the Cornell Nanoscale Science and Technology Facility(NNF-CNF) at Cornell University.

In this process, the temperature and deposition rates are chosen toallow the poly-silicon to fill the etched grating structure and the sizeof the crystalline regions in the poly-silicon are varied by controllingthe growth conditions, which in turn varies the refractive index of thefill material accordingly. This and related filling techniques (e.g.,sputtering, thermal evaporation, etc.) provide the ability to fine tunethe refractive index modulation of the volume transmission gratings inorder to optimize peak diffraction efficiency and spectral bandwidth.

Each of the 16 gratings 610 in the array has a unique combination ofspatial period (10 or 20 μm) and modulation duty cycles ranging from 10%to 90%. This experiment shows the feasibility of using depositiontechniques to optimize the refractive index modulation inlithographically fabricated LWIR diffraction gratings to achieve thedesired high efficiency, broadband characteristics.

Filling Deep-Etched Grating Structures Using LP-CVD

In order to produce high efficiency volume transmission gratings in theLWIR band, there must be a procedure for producing a controllablerefractive index modulation with the proper spatial period throughout adeep enough region to bring the structure into the Quasi-Bragg regime.The depths over which the refractive index modulation needs to bemaintained ranges from tens to hundreds of μms depending on the spatialperiod and wavelength in the medium. Experimental evidence was obtainedthat clearly shows the feasibility of filling very deephigh-aspect-ratio diffractive structures with materials of varyingrefractive index. This feasibility comes in two parts: First thefeasibility of making high-aspect ratio grating structures that are tensto hundreds of μms deep; and second, the feasibility of filling suchdeep structures with materials of the desired refractive index.

We obtained experimental evidence that clearly demonstrates thefeasibility of filling the deep-etched structures as shown in FIG. 18with poly-silicon or poly-germanium using Low Pressure Chemical VaporDeposition (LP-CVD). FIG. 22 is a photomicrograph of a cleaved silicongrating in cross-section that originally looked much like that shown inFIG. 18 , but was then subject to the deposition of a poly-silicon layerusing LP-CVD.

The period of the grating illustrated in FIG. 22 is 20 μm, and thesilicon grating facets are now coated with a layer of poly-silicon. Byvarying deposition parameters such as the temperature and reactant flowsused in the LP-CVD, the deposited silicon can range anywhere fromcompletely amorphous to poly-silicon with varying degrees ofcrystallinity. This variation in crystallinity is accompanied by avariation in the refractive index of the deposition material withrespect to the host wafer material. Furthermore, as an alternativepoly-germanium can be deposited in place of the poly-silicon and therefractive index can be tuned using annealing-induced diffusion oralloying. The structure visible in the silicon wafer cleaved surfacebelow the grating may be evidence of stress related to the deposition,which can be reduced with either annealing and/or parameter variations.

The close-up photomicrograph of the coated grating illustrated in FIG.22 shows where the superimposed red line profile 633 outlines the largeopen-duty-cycle of the uncoated silicon grating structure. A keyobservation is that the deposited material coats the silicon gratingfacets 634 very evenly all the way down the depth of the facet. This isparticularly important in order to avoid the “pinching off” of thedeposition that would result if the deposition rate is higher on the topsurface of the facts than deep in the trenches. Part of this uniformityis accomplished by controlling the temperature during deposition toallow the deposited atoms mobility to evenly spread over the surface.

FIG. 23 illustrates another 20 μm period silicon grating that has beensubjected to the same deposition process, but in this case the gratinghas a smaller open-duty-cycle than that of the grating shown in FIG. 22, such that the silicon coating fills nearly the entire grating exceptfor a very thin gap. The close-up photomicrograph of this coated gratingshows where the superimposed line profile 634 outlines the mediumopen-duty-cycle of the uncoated silicon grating structure. Thismagnified view illustrates the tremendous utility of the LP-CVDtechnique, as the original silicon grating facets 645 are uniformlycoated leaving a tiny gap in tact all the way to the bottom of thefacet. This is a strong demonstration of the feasibility for making verydeep, high aspect grating structures.

FIG. 24 illustrates a close-up photomicrograph of yet another 20 μmperiod silicon grating that has been subjected to the same depositionprocess, but in this case the grating has a still-smalleropen-duty-cycle than those of the gratings shown in FIG. 22 , such thatthe silicon coating now fills the entire grating structure and evenovercoats the grating on top. The superimposed line profile outlines thesmall open-duty-cycle of the uncoated silicon grating structure wherethe wide silicon grating facets 654 are clearly visible extending upfrom the silicon wafer substrate. In between the silicon facets, it canbe seen that the gaps 656 are uniformly filled with poly-silicon leavingno apparent voids. In addition, the poly-silicon is seen to overfill thegap regions and form a uniform layer on top 657 of the grating. Thistype of uniform layer coating on the grating could be very useful forfurther depositing an antireflective coating layer.

FIG. 25 is a bright-field photomicrograph 670 of a 10 μm spatial periodgrating that was subjected to a LP-CVD poly-silicon deposition. Sincethe etch depth of this grating is similar to that of the gratingsillustrated previously. This smaller spatial period results in anuncoated silicon grating structure that has a much higher aspect ratio.Even still, it is apparent from this illustration that these deep voids674 have been uniformly filled with the poly-silicon leaving no apparentvoids, and that there is an additional coating 677 of poly-silicon overthe entire grating. The poly-silicon fill and cap material ishighlighted under this bright-field illumination. Similarly, FIG. 26 isa dark-field photomicrograph of the same grating and highlights theoriginal silicon grating facets that have been filled and over-coated.

Since silicon begins to absorb at the long end of the LWIR spectralband, it is desirable to minimize the amount of silicon in the volumetransmission grating. While an identical fabrication cycle usinggermanium wafers and poly-germanium LP-CVD deposition could beattempted, several other fabrication processes could also be pursued.For example, poly-germanium could be deposited in the silicon gratingand then the silicon wafer substrate polished off. This would leave agrating that is all germanium except in the modulation region. Thisregion need be only tens to hundreds of μms thick, and there may onlyneed to be a small duty cycle, e.g., 10-20% of silicon, to provide theappropriate modulation. As a result, this small amount of silicon maynot pose a significant source of system loss, and annealing thestructure can result in a varying silicon-germanium alloy with increasedLWIR transmission characteristics.

Feasibility Experiments Supporting Fabrication by Molding

The feasibility of fabricating gratings using molding techniques as apossible compliment to the LP-CVD techniques described above wasexperimentally demonstrated. These techniques may be useful for makingor filling germanium gratings, or for inexpensively adding a substrateon top of germanium gratings formed with LP-CVD deposition into silicongratings where the silicon substrate is subsequently polished off. Thefirst step in these experiments was to cleave one grating sample fromthe silicon grating-array wafers that were described above. One of thesmall test gratings obtained from cleaving the wafer is shown in FIG. 27.

Fused silica test chambers were designed and built to house the gratingtest samples. These test chambers were designed to hold a vacuum at atemperature of 1100° C., and were connected to a gas mixing and pumpingmanifold that allows for evacuation of the vessel and for back-fillingof the vessel with argon gas. The small test chamber 730 is shown insidea furnace 720 in FIG. 28 To fabricate the molded germanium grating, thegrating sample was placed in the test chamber with small quantities ofpurified germanium. The sample chamber was then heated to approximately1000° C. in an atmosphere of Argon, where the germanium was melted andpartially filled the voids of the silicon test grating. The moldedgermanium ingot was then separated from the silicon grating. While thedeep patterning of the germanium sample was not expected in this firstattempt, a sharp grating structure was indeed transferred into thegermanium ingot.

FIG. 29 contains a photomicrograph 800 that clearly illustrates the 20μm period grating structure molded into the germanium. The feasibilityof this technique is shown in this experiment, and the molded gratingappears deeper, although not matching the silicon master in depth. Itwas also observed that some molten germanium flowed under the interfacebetween the tube and the silicon grating that it was resting on. Insubsequent work, surfactants and flux-like reactants can be used todecrease surface tension and allow more complete filling of the siliconstructures.

The experimental gratings demonstrate the feasibility of producing LWIRvolume transmission gratings that can be optimized for high efficiencyand broad spectral bandwidths. While the grating development wasoriented toward reducing the fabrication risks of LWIR transmissiongratings, it is expected that the characterization and optimization worknecessary to produce volume transmission gratings specifically optimizedfor the high diffraction efficiencies and broad spectral bandwidthsrequired for the compact, lightweight LWIR hyperspectral imaging sensorscan be performed.

Although the invention has been described with respect to variousembodiments, it should be realized this invention is also capable of awide variety of further and other embodiments within the spirit andscope of the invention.

The invention claimed is:
 1. A volume diffraction grating comprising: a structure comprising a number of protrusions and a number of cavities; the structure comprising substantially a first material; the number of protrusions being at least partially filled by at least a second material; the volume diffraction grating being a spatially varying grating with a varying combination of the first and second materials;-refractive index modulation for the volume grating tuned by at least one of annealing induced diffusion or alloying; said refractive index modulation, after tuning, being such as to provide predetermined diffraction efficiency of said volume diffraction grating.
 2. The volume diffraction grating of claim 1 wherein the first material is silicon and the second material is germanium.
 3. The volume diffraction grating of claim 1 wherein the first material is silicon and the second material is polysilicon.
 4. The volume diffraction grating of claim 1 wherein the volume diffraction grating is a volume transmission diffraction grating.
 5. The volume diffraction grating of claim 1 wherein the volume diffraction grating is a volume reflection diffraction grating. 